8. They are soured in very dilute hydrochloric acid;[220] and then left on
'stillages'[221] for 5 or 6 hours.
[Footnote 220: Sp. gr. 1·0125, or 2-1/2° Twaddle.]
[Footnote 221: Low stools or props to keep them from the ground.]
9. They are, finally, thoroughly washed, well squeezed between rollers,
dried over steam-heated tin-cylinders, starched or dressed, and finished.
This is the usual process for good calicos, Muslins, and other light
goods, are handled rather more carefully; whilst for commoner ones the
sixth and seventh operations are generally omitted. The whole usually
occupies 5 days; but by using Mr Barlow's high-pressure steam kiers, it
may be performed in two days. Yarns, &c., may be bleached in a similar
manner by first looping the skeins together.
_Obs._ According to the most reliable authorities, the strength of
cotton-fibre is not impaired by its being boiled for two hours in milk of
lime, under ordinary pressure, out of contact with the air; nor, according
to the bleachers, even by sixteen hours boiling at the strength of 40
_lbs._ per 100 galls. It is said that lime is less injurious than 'soda.'
Solution of caustic soda, sp. gr. 1·030, does not injure it, even by
boiling under high pressure; but, in practice, soda-ash, or carbonate of
soda, is used, and this only in the second bucking, and in the third, if
there be one. The strength now never exceeds 25 _lbs._ of the crystals to
the 100 _galls._, and is usually less.
Experiments have shown that immersion for 8 hours in a solution of
chloride of lime containing 3 _lbs._ to the 100 galls., followed by
souring in sulphuric acid of the sp. gr. 1·067, or for 18 hours in acid of
1·035, does not injure it.
By the improved method of previously treating the goods with lime or
alkalies, little chloride of lime is required. Indeed, it is said that
where 300 _lbs._ were formerly employed, 30 to 40 _lbs._ only are now
used. At the same time it is right to mention, that though a solution at
1/2° Twaddle is usually regarded as the best and safest strength, yet in
some bleach works, particularly for inferior and less tender goods, this
is greatly increased, even up to 5°, the period of immersion being
proportionately reduced, as it is not safe to expose the goods long to the
action of such powerful solutions. With the higher strengths they are
passed rapidly through the liquid with the calender, sufficient time only
being allowed to soak them thoroughly; then immediately through the acid
or souring, followed by washing as before.
In Scotland and Ireland the washing is generally performed by wash-stocks;
whilst in Lancashire, dash-wheels, or washing machines with squeezers, are
almost always used for the purpose.
Cotton loses about 1-20th of its weight by bleaching.
II. BLEACHING of =Linen=:--Linen may be bleached in a similar way to
'cotton,' but the process is much more troublesome and tedious, owing to
its greater affinity for the colouring matter existing in it in the raw
state. Under the old system, several alternate buckings with pearlash or
potash and lengthened exposure on the field, with one or two sourings, and
a final scrubbing with a strong lather of soft soap, constituted the chief
details of the process. In this way a high degree of whiteness, though not
an absolutely pure or snow white, was ultimately produced. Grass-bleaching
or crofting is still extensively used for linen; but it is more generally
employed only for a limited time, and in combination with a modification
of the system at present almost universally adopted for cotton goods;
whilst, in some cases, crofting is omitted altogether, and the bleaching
conducted wholly by the latter process. The following _Tables_ exhibit the
outlines of the new system as at present practised in Ireland and
Scotland:--
_a._ For plain sheetings:--
1. They are bucked for 12 or 15 hours in a lye made with about 1 _lb._ of
pearlash (or soda-ash) to every 56 _lbs._ of cloth, and washed.